Forests

Ecosystem Services

It is very likely that climate change will decrease the ability of many forest ecosystems to provide important ecosystem services to society. Tree growth and carbon storage are expected to decrease in most locations as a result of higher temperatures, more frequent drought, and increased disturbances. The onset and magnitude of climate change effects on water resources in forest ecosystems will vary but are already occurring in some regions.

The Millennium Ecosystem Assessment106 defines four categories of ecosystem services: supporting, provisioning, regulating, and cultural. Recent studies have focused on defining and quantifying the full range of services provided by forests including recreation, wildlife habitat, biodiversity, cultural values, and non-timber forest products.107,108 Here, we focus on climate change effects on two of the most important forest-based services: forest carbon dynamics (regulating and provisioning) and forest water resources (regulating and provisioning). (For additional discussion on the effects of climate on ecosystem services, see Ch. 7: Ecosystems and the regional chapters.)

Forest Carbon Dynamics

Forest productivity (Key Message 1) is one of many factors that determine carbon storage potential.109 Typically, soil carbon is the largest and most stable carbon pool in forest ecosystems,14,110,111,112 but increased above-ground biomass production in forests is not necessarily accompanied by higher soil carbon content. In some locations, heavy rainfall events will result in flood-related tree mortality, leading to soil erosion and losses of particulate and dissolved organic carbon from forests.113 Increased disturbances such as harvesting, wildfire, and insect and disease damage can also release carbon stored in soils, especially where multiple disturbances occur over a short time span (Figure 6.6).114

The fate of carbon in forests depends, in large part, on the type, extent, frequency, and severity of the disturbance.114,115 Severe disturbances, such as stand-replacing wildfire, typically result in the immediate release of carbon to the atmosphere,32 a reduction in stand productivity, the transfer of carbon from live to dead pools, and an increase in decomposition.114,115 Productivity will gradually increase following a disturbance, and decomposition will decrease as the forest recovers. The abrupt release of carbon after a disturbance transitions to net carbon uptake through forest regrowth. However, the full effect of the disturbance on atmospheric CO2 depends on the timing of disturbance-induced CO2 releases. Although carbon storage in biomass will increase in areas where tree growth rates rise, those increases will be small compared to the reduced storage that occurs in response to more disturbances.18

Figure 6.6: Forest Disturbances Across the United States

Figure 6.6: This figure shows the cumulative area of disturbed forestland across the contiguous United States for 1984–2014. The small boxes illustrate how disturbances differ regionally. Data for Alaska, Hawai‘i and the U.S.-Affiliated Pacific Islands, and the U.S. Caribbean regions were not shown on the original map from the published source. Source: adapted from Williams et al. 2016.114

Economic and population growth will affect land-use decisions that influence forest-based carbon storage. Over the last several decades, conversion of forestland to other land uses has contributed to CO2 emissions,14,116 and this trend is likely to continue, although this is among the most significant sources of uncertainty in the forest carbon sink in the United States.18,117,118 The current (2017) U.S. deforestation rate (the conversion from forest to nonforest land use) of 0.12% per year is more than offset by forest gain from afforestation (the establishment of a forest where there was no previous tree cover) and reforestation, for a net gain of forest area of 0.09% per year (679,000 acres).14 Gains occur mostly through a transition from grasslands and croplands to shrublands, woodlands, and forests, and losses occur mostly in urban areas (see Ch. 5: Land Changes for details on forest land-use trends).14 While some individual states have lost forestland, overall, each region of the United States (for example, northern, southern, Rocky Mountain, and Pacific coast) has gained forestland area over the past 20 years.14,16

Net storage of atmospheric carbon by forests (742 teragrams, or Tg, of CO2 per year from 1990 to 2015) has offset approximately 11% of U.S. CO2 emissions.14 Assuming no policy intervention—and accounting for land-use change, management, disturbance, and forest aging—U.S. forests are projected to continue to store carbon but at declining rates (35% less than 2013 levels by 2037) as a result of both land use and lower CO2 uptake as forests grow older.15,16,17,18,42

Although forest area has increased over the last few decades (Ch. 5: Land Changes, Figure 5.1), this trend is projected to level off by 2030, then decline gradually as human population expands and afforestation on agricultural lands slows,18,42 with more rapid leveling in the West compared to the East. However, carbon accumulation in surface soils (at depths of 0–4 inches) resulting from reforestation activities can help mitigate declining carbon storage in U.S. forests over the long term. Surface soils in reforested areas are currently accumulating 13–21 Tg carbon per year, with the potential to accumulate hundreds more Tg of carbon within a century.112,119

Economic and population trends will affect national and global production and consumption of wood products, which can temporarily store carbon. The storage of carbon in and emissions from wood products contribute to carbon stores and exchanges with the atmosphere; the carbon stored in wood products accumulates as wood is harvested from forests at a rate that exceeds carbon releases from the decay and combustion of wood products already in use. The harvested wood products pool alone is not a direct sink for atmospheric carbon, but losses from the pool are a direct source of atmospheric carbon. Although the contribution of harvested wood products is uncertain, the worldwide net surplus of carbon in wood products is estimated to be approximately 8% of the established global forest sink (189 Tg carbon per year).120 In the United States, 76% of the annual domestic harvest input to the wood products pool in 2015 (110 Tg carbon per year) was offset by release processes (84 Tg carbon per year), resulting in an increase in wood products of 26 Tg carbon.14

Forest Water Resources

Forested watersheds provide water for municipal water supplies, agricultural irrigation, recreation, spiritual values, and in-stream flows for aquatic ecosystems. Changes in snowfall amount, timing, and melt dynamics are affecting water availability and stream water quality. In the western United States (especially the Pacific Northwest), less precipitation is falling as snow and more as rain in winter months, leading to a longer and drier summer season (Ch. 24: Northwest).121 Persistence of winter snowpacks has also decreased in the northeastern United States over the last few decades, with more mid-winter thaws (Ch. 18: Northeast). Changing snowmelt patterns are likely to alter snowmelt contributions to the flushing of soil nutrients into streams in both western122 and eastern forests.123

Forest watersheds moderate the effects of extreme climate events such as drought and heavy rainfall, thus minimizing downstream impacts on aquatic ecosystems and human communities such as flooding, low flows, and reduced water quality. Disturbances and periodic droughts affect streamflow and water quality,12,13,124 as do changes in forest structure that are influenced by climatic variability and change, such as leaf area and species distribution and abundance.33 For example, drought-related bark beetle outbreaks and wildfire kill trees, reducing water uptake and evapotranspiration and potentially increasing water yield,125 although water yield can decrease if regrowing species have higher water-use demands than did the insect- or fire-killed trees.126

Wildfires can also increase forest openness by killing midstory and overstory trees, which promotes earlier snowmelt from increased solar radiation. This, in turn, leads to more winter runoff and exacerbates dry summer conditions, especially in cooler interior mountains.127,128 In warmer forests, typically in wetter climates where wildfire is currently rare, increased forest openness can in some cases increase snowpack retention.129 Wildfires can increase erosion and sediment in western U.S. rivers,130 as well as reduce tree cover adjacent to rivers and streams and thus increase stream temperature.131,132 In eastern U.S. forests, the proportion of tree species with moderate water demands (mesophytes) is increasing in many areas as a result of fire exclusion, less logging and other disturbances, and possibly a warmer climate.133,134 Mesophytes transpire more water than other species occupying the same area, thus reducing streamflow.135,136